1. Field of the Invention
The present invention relates to a nitride semiconductor light-emitting device as well as an optical device and a light-emitting apparatus that use the nitride semiconductor light-emitting device. In particular, the present invention relates to a nitride semiconductor light-emitting device having a high emission efficiency and to an optical device and a light-emitting apparatus with that nitride semiconductor light-emitting device.
2. Description of the Background Art
Group V element such as As, P or Sb is added to gallium nitride-based compound semiconductor. Then, a resultant mixed crystal exhibits a great change in the bandgap with respect to a small change in the lattice constant. In consideration of this phenomenon, the amount of As, P or Sb to be added to the gallium nitride-based compound semiconductor, which constitutes a light-emitting layer, can be varied to change the emission wavelength. For example, Japanese Patent Laying-Open No. 10-270804 discloses a semiconductor laser device having, on an a-plane sapphire substrate, a non-doped GaN0.94As0.06xe2x80x94GaN strained quantum well active layer (each thickness: 5 nm, 3 cycles), and a semiconductor laser device having, on an a-plane sapphire substrate, a non-doped GaN0.97As0.03xe2x80x94GaN strained quantum well active layer (each thickness: 5 nm, 5 cycles).
For the light-emitting layer constituted of a GaNAs crystal, GaNP crystal or GaNSb crystal, the effective mass of electrons and holes can be decreased relative to conventional InGaN crystals. This suggests that the low carrier density can produce population inversion for a lasing action (decrease in the lasing threshold current value). However, if As for example is added to a light-emitting layer constituted of nitride semiconductor, it is highly likely that the resultant light-emitting layer is separated into a region with a high nitrogen concentration and a region with a high As concentration (this phenomenon is hereinafter referred to as xe2x80x9cconcentration separationxe2x80x9d). Moreover, the light-emitting layer could be separated into different crystal systems, i.e., hexagonal system in the region of the high nitride concentration and cubic system in the region of the high As concentration (hereinafter referred to as xe2x80x9ccrystal system separationxe2x80x9d). Such a separation into different crystal systems causes decrease in the emission efficiency due to the deteriorated crystallinity. The crystal system separation could be caused not only by As but also by P or Sb added to the nitride semiconductor light-emitting layer. It is thus desired to avoid the crystal system separation for enhancing the emission efficiency (emission intensity).
One object of the present invention is to clarify a structure of a light-emitting device that can improve the performance of the light-emitting device which includes a light-emitting layer constituted of nitride semiconductor containing at least one of As, P and Sb, and to provide the light-emitting device that has thus an enhanced emission efficiency or emission intensity or has a lower threshold current density.
A nitride semiconductor light-emitting device according to the present invention includes a substrate, an n-type nitride semiconductor layer and a p-type nitride semiconductor layer formed on the substrate, and one or a plurality of well layers provided between the n-type nitride semiconductor layer and the p-type nitride semiconductor layer. The well layer is constituted of nitride semiconductor containing at least N and element X including at least one element selected from the group consisting of As, P and Sb. The nitride semiconductor of the well layer has at most 30% in atomic percent represented by expression {NX/(NN+NX)}xc3x97100 where NX represents the number of atoms of element X and NN represents the number of atoms of N. The well layer has a thickness ranging from 0.4 nm to 4.8 nm. This composition and thickness of the well layer can reduce the threshold current density or enhance the emission intensity.
For the light-emitting device according to the present invention, preferably the well layer contains at least one element selected from the group consisting of Si, O, S, C, Ge, Zn, Cd and Mg in a concentration ranging from 1xc3x971017/cm3 to 1xc3x971019/cm3. Such impurity elements can be added to improve the crystallinity of the well layer and to further decrease the threshold current density or further enhance the emission intensity.
The light-emitting device according to the present invention typically includes a barrier layer contacting the well layer. Preferably the barrier layer has a thickness ranging from 3 nm to 20 nm. The barrier layer with such a thickness can prevent slight crystal system separation occurring in the well layer from propagating to other well layers.
The light-emitting device according to the present invention includes one or a plurality of well layers. Preferably, the number of the well layers is at most 8. The nitride semiconductor light-emitting device including an appropriate number of well layers can further be reduced in the threshold current density or increased in the emission intensity.
For the nitride semiconductor light-emitting device according to the present invention, preferably the substrate is a nitride semiconductor substrate or pseudo nitride semiconductor substrate detailed below.
The nitride semiconductor substrate can be used for the light-emitting device of the present invention in order to lessen crystal system separation which could occur in the well layer. Here, the nitride semiconductor substrate refers to a substrate made of nitride semiconductor and having a thickness suitable as the one on which components of the device are deposited. The nitride semiconductor substrate includes a substrate which is formed of nitride semiconductor crystals produced by various crystal growth methods and processed into an appropriate size, as well as a substrate formed by growing a nitride semiconductor crystal layer on another crystal material and then removing this another crystal material. Typically, the nitride semiconductor substrate is a substrate constituted of at least AlxGayInzN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61, x+y+z=1). In the nitride semiconductor substrate, approximately 10% or less of nitrogen constituting the nitride semiconductor substrate may be substituted with at least one of As, P and Sb to the degree that the crystal shape of hexagonal system is maintained. In addition, at least one of impurity elements Si, O, Cl, S, C, Ge, Zn, Cd, Mg and Be may be added to the nitride semiconductor substrate. In particular, for the purpose of providing n-type conductivity to the nitride semiconductor substrate, preferably any of Si, O and Cl among the impurity elements is added in an amount ranging from 3xc3x971017/cm3 to 1xc3x971019/cm3.
The pseudo nitride semiconductor substrate herein refers to a substrate structured by growing a nitride semiconductor crystal layer on another crystal material. Typically the nitride semiconductor is represented by AlxGayInzN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61, x+y+z=1). A particularly preferable nitride semiconductor is GaN. The pseudo nitride semiconductor substrate, particularly pseudo GaN substrate can be used to lessen crystal system separation which could occur in the well layer. The pseudo nitride semiconductor substrate includes a pseudo GaN substrate as shown in FIG. 2 and a pseudo GaN substrate as shown in FIG. 3A, for example, that are described below. The former pseudo GaN substrate is constituted of at least a nitride semiconductor crystal layer, a seed substrate on which the nitride semiconductor crystal layer is grown, and an anti-growth layer on which the nitride semiconductor crystal layer is not directly grown. The latter pseudo GaN layer is formed by etching a substrate or nitride semiconductor layer to form trenches therein and thereafter covering the trenches with a nitride semiconductor crystal layer.
The light-emitting device according to the present invention typically includes a light-emitting layer formed of a well layer or a light-emitting layer formed of a well layer and a barrier layer contacting the well layer. For example, if the light-emitting layer has a single quantum well structure, the light-emitting layer is constituted of a well layer only or constituted of a structure: barrier layer/well layer/barrier layer. If the light-emitting layer has a multiple quantum well structure, the light-emitting layer is constituted of a structure: barrier layer/well layer/barrier layer . . . /well layer/barrier layer, or well layer/barrier layer/well layer . . . barrier layer/well layer/barrier layer. Here, the bandgap energy of the well layer is smaller than the bandgap energy of the barrier layer.
Further, according to the present invention, an optical device or a light-emitting apparatus including the above-discussed nitride semiconductor light-emitting device is provided.
It is noted that an atomic percent (%) represented by expression {NX/(NN+NX)}xc3x97100, where NX represents the number of atoms of element X and NN represents the number of atoms of N, is herein referred to as xe2x80x9catomic percent of element X.xe2x80x9d
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.